When you have an empty gear slot that displays the word "Find," it means that particular gear piece must be acquired. Tap the gear slot, and you will be taken to the GEAR screen with further information on what gear piece is needed and how many. Select the gear in question and tap the Find button on the right-hand side. At the newly opened screen, you will see an array of options where you can acquire said gear piece. These options can range from an orb purchase, special store offer, or Campaign missions where that gear piece can be acquired.